Pilot-wire-protective system



H. C. GRAVES, JR

PILOT WIRE PROTECTIVE SYSTEM June 17, 1930.

5 sheets-sheet i -Filed Nov. 2l, 1927 ,my FH.

INVENTOR Herbe/f C.' 6m Ve: Jv

ATTORNEY June 17, 1930. H. c. GRAVES, .1R

PILOT WIRE PROTECTIVE SYSTEM Filed Nov, 2l, 1927 5 SheetSSheet 2 INVENTOR Nr NI 'V' nl.,

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ATORNEY June 17, 1930. H. c. GRAVES, JR 1,764,297

PILOT WIRE PRoTEcTrvE SYSTEM Filed Nov. 21, 19'27 5 sheets-sheet 3 June 17, 1930. H. c. GRAVES, .1R 1,764,297

- PILOT wIRE PROTECTIVE SYSTEM Filed Nov. 21, 1927 5 Sheets-Shet 4 Nan/h fra-.g

A'TTORNEY June 17, 1930, H. c. GRAVES, JR 1,764,297

PILOT WIRE PROTECTIVE SYSTEM vFiled Nov. 21, 1927 5 Sheets-Sheet 5 /Va//r l life:

Herbe/*f C Grnen/f.

ATTORNEY Patented June 17, 1930 UNITED STATES PATENT OFFICE HERBERT C. GRAVES, JR., OF VERONA, PENNSYLVANIA, ASSIGNOR T WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA kPILOT-WIEtE-PROTECTIVE SYSTEM Application, filed November 21, 1927.

This invention relates to pilot-wire protective systems and circuit-breaker control systems, and particularly to pilot-wire protective systems of the circulating-current tyre The principal objects of this invention are as follows:

To provide a pilot-wire protective system and a circuit-breaker control system of the circulating-current type.

To provide a protective system which shall he relatively sensitive to fault conditions on the ypower system.

A protective system utilizing an inductiondisc-type differential relay of dual sensitivity.

A protective system utilizing a differential relay in the neutral circuit to provide greater sensitivity in case of a ground fault on the power system.

A protective system wherein the voltampere burden on the main-line current transformers` is of relatively small magnitude.

A protective system wherein the potentials applied to thc secondaries of the main-line current transformers are of relatively small magnitude.

A protective system wherein the relays may he so adjusted that an open circuit in the pilot-wire system shall not cause false interrliption of the main lines, under normal conditions of operation, lout which shall still he effective to interrupt the main lines when r a fault condition occurs relative to the main vide a relatively perfect. balance for any type Serial No. 234,648.

of power system through fault and yet which need not be so precisely balanced as in other systems heretofore used.

A protective system utilizing an impedance device or resistance means in the neutral circuit for the purpose of obtaining relatively great sensitivity and selectivity.

A protective system in which the currenttransfornicr characteristics at the two ends of a line to be protected need not be exactly alike and yet provide a system which shall loe of such selectivity that only a faulty section will he isolated and the .non-faulty remain uninterrupted.

A protective system in which the effect of potentials induced in the pilot-wire system is negligible.

A protective relay for a pilot-wire protective system in which the characteristics of the relay are such that the differential current required to cause actuation of a circuit interrupter is of relatively small magnitude when the direction of current in one of the relay coils is reversed relative to the direction of the current in the other coil of the saine relay.

A protective system which shall automatically become more sensitive to the smaller value of fault current when the magnitudes of the fault currents fed from the two ends of the section are different.

A protective relay which, when located at the end of the faulty section which is supply ing the smaller value of fault current7 automatically loeconies more sensitive.

A protective system which shall isolate ground faults with relatively great rapidity and which shall be of relatively great sensitivity to ground faults.

To provide a means connected in the neutral circuit which may he so adjusted as to permit its associated relays to be set very sensitively without faulty operation when a fault current from some other part of the power system passes through the section protected.

To provide a protective relay system which shall be responsive only to excess current passing` through its associated balancing im pedance devices a result of lineto-line or line-to-ground faults.

A protective system which shall permit such adjustment oic the current division as to prevent false opera-tion of circuit interrupters, due to changes in the impedances ot the pilot wires not unreasonably large.

A protective system 'for a three-phase power system utilizing tour impedance devices or resistors associated with each end ot the section to be protected, 'for compensating for all types of power-system faults.

To provide a protective relay, the pilot Wire coil ot which is a restraining coil and not an operating coil.

A pilot-pire protective system having relays responsive to a .flitlicrential cu rent between an operating coil which is connected to a balancing resistor circuit and a restraining coil .vhich is connected to a pilot-Wire circuit.

There are several objections to the pilot- \vire protective systems heretofore used, the principal objections or disailvantages ot which are as follows, and all of which are obviated by this invention:

Heretofore, there have been rather narrow limits to the length of the pilot Wire that could be used because the sensitiveness of the protective system decreased as the length Was unduly increased unless the size ot the pilot ivire was increased by a prohibitive amount. In this invention, the length ot power line to be protected may be materially increased because otl the smaller burden it imposes upon the current transformers tor the same length of pilot ivires.

Hcretot'ore, an open circuit in the pilot ivire resulted in tripping the circuit breakers. ln this invention, an open circuit in a pilot ivire doc-s not result in such false opera tion ot the circuit breakers, but an indication will, nevertheless, be given as to the existence et such an open circuit in a pilot Wire. rfhis effectis obtained in the following manner. An open circuit in a pilot ivire will cause the total current transformer current to lioiv through the operating coil ol the relay. However, the characteristics of the relay are such that, when current tlovrs through one coil only, current equal to tvvice the value of the relay current tap used is required in order to cause operation. By using a current tap, the value ot' which is equal to somewhat more than one-halt of the full-load current, the relay ivill still be ineffective if a pilot Wire is opened. lhenever a fault occurs, either Within or ivithout the section of line on which these relays are installed, the increased magnitude of the current .vill cause the relays to operate.

Heretotore, differences in the charging currents at the tivo ends ot the line might cause false tripping of the circuit breakers when a fault ot large magnitude occurred in some other section of the power system. lith this invention, such charging currents will not ,anse false interruptions of the non-faulty poiver sections because the balancing resistors can be adjusted so that there is a slight normal restraint for those installations Where the characteristics oit' the power system are such as to make possible considerable differences in charging current at consecutive stations.

Heretofore, induced voltages from neighboring circuits might falsely trip the circuit breakers ot the protected section of the power system. lith this invention, such induced voltages will not cause such false operation because the current due to the induced voltage ivill flow through the two relay coils in series. Since each coil will still carry the same current, the relays Will remain inoperative or inetl'ective.

Heretofore, in circulating current schemes. each main-line current transformer normally had loiv voltage on its secondary, but, in case of a through short circuit or While the` circuit breaker was being opened in isolatingr a local fault, the voltage might become of large magnitude. lith opposing-voltage schemes, the voltage on the current transtormers is normally very high. lith this invention, such high vdtages do not occur upon the secondaries ot the main-line current transformers because of the relatively loiver impedance of the relay With its associated resistor. For the ordinary circulating-current scheme, it was necessary to make either the impedance of the relay circuit, or the relay current tap setting, so high that suliicient current to operate the relay would not flow through the relay circuit even When a 'fault outside of the protected section caused high values ot current to exist. Thus, the ordinary circulating-current scheme either sacriliced sensitivity because of the high current tap setting or caused a high voltage to exist on the currenttransformer terminals because ot' the high impedance ot the relay circuit.

Heretofore, a balancing' neutral resistor or impedance device connected in the neutral circuit to the ground pilot Wire has not been utilized, and, consequently, faulty isolation ot a local non-faulty sect-ion was liable to occur When a fault occurred in some other section ol the power line unless the sensitivity ot the relays Was considerably reduced by increasing the resistance in the balancing resistors. This resulted in producing an overbalanced restraining torque on the relay under normal conditions. lith this invention, however, by the utilization of such a balancing neutral resistor, the relays may be adjusted very sensitively and yet, at the same time, a perfect selectivity may be obtained to isolate the faulty section only and leave undisturbed the non-faulty section of the power system. In other words, the neutral resistor causes the control current to divide equally between the two relay coils for any type of fault which occurs outside of the protected section, and permits the relays to be set very sensitively without resulting in false operation of the circuit breakers.

lleretofore, relays have been used which operated upon an imbalance of the current in the protective system and which required a precise setting of the adjusting resistors associated with the relays. Vith this invention, however, a relay is utilized which op.- erates only upon an excess current through the adjusting resistors and, consequently, it is possible to avoid the exact precision heretofore required in the setting of resistors. 1f the pilot-wire impedance is known only approximately or if Ait will increase app-reciably, due to temperature or for any other reason, the value of the balancing resistor is Iliade slightly higher than the value which would be used if a definite and constant value for the pilot-wire impedance were known. This does not cause the relay to operate or tend to operate, but ymerely places a slight overbalanced restraint upon it.

This is a pilot-wire protective system of the divisionalcirculating-current type. Means having a neutral connection, in the form of the usual current transformers, are provided for obtaining a control current proportional, in direction and magnitude, to the current flowing in the main power conductors. Such current transformers for a three-phase power system are preferably three'in number at each end of the power line section to be protected.

Phase pilot wires are provided and are associated with veach of the main power-line conductors or phases and also an additional pilot wire, called a ground pilot wire, is provided, all of the pilot wires running the length of the power section to be protected. Associated with each of the main conductors or phases is a balancing circuit in which is located a resistor or impedance device for balancing the division of control currents between the pilot wire and the balancing circuits.

In addition to the impedance devices already mentioned, is provided another special impedance device connected to vthe neutral connection of the current transformers and also to the balancing circuits. The purpose of the special impedance device is principally to so arrange the distribution of the control currents, produced by a fault in a section adjacent to or other than the section under protection which is non-faulty, that faulty operation of the circuit interrupters in the nonfaulty section is `prevented. A relay responsive to the division in control vcurrents or tection of parallel lines.

'the line-current transformers is relatively small. When applied to a three-phase power system, this scheme is novel in that four balancing resistors or impedance devices are utilized instead of three, with the result that balanced conditions are maintained, irrespective of whether a through fault or a fault outside of the protected section, is phase-tophase or phase-to-ground. This permits the use of ground relays having a low-current setting, thus increasing the sensitivity of the protection. However, for applications where ground-fault protection is not desired, the ground relays, but not the neutral. balancing resistors, may be omitted.

lf the usual ten-to-one ratio insulating current transformers connected in the pilot wires be utilized, this protection may be applied to lines of relatively great length because of the effective low impedance of the pilot wires. For shorter lines where the pilot-wire impedance is of relatively small magnitude, the insulating current transformers may be omitted unless they are required for protection against high voltages. With insulating current transformers of the ratio named, each of the line balancing resistors may be equal to 1/20() of Z, the pilot wire impedance, plus the impedance of the insulating current transformers; but the neutral balancing resistors should be equal to 1/200 Z only. f

Another novel feature of this protective system is in the characteristics of the selective differential relay which is utilized. The relay designis such that, when a fault occurs in a section protected by this pilot-wire scheme, the circuit breaker, through which the maximum current is flowing, will automatically operate on one sensitivity characteristic of the relay and that through which the minimum current is flowing will 'automatically be responsive to the relay characteristic of greatest sensitivity. As a result, that breaker which would have the least tendency to trip, in arrangements heretofore used, operates in this system on the most sensitive characteristic of the relay.

The dual-sensitivity characteristics of the selectiif'e differential relay utilized in this protective system makes it suitable for pro- When the operating conditions of the relay are such that the currents flowing through its two coils are in the same direction, the indications are that the iso fault is near the other end of the power line, and the relay operates on its least sensitive characteristic, as shown in curve A of Fig. 3. viVhen the currents through the two coils of the relay are in opposite directions, the indications are that the fault is close to that particular relay. This relay then operates on its sensitive characteristic shown in curve B of Fig. 3 and thus permits the relay to be installed at stations where only a. small amount of feedback is available. lf the relay operations at both ends of the line are not simultaneous, the opening of a breaker by one set of relays permits the other set of rela-ys to operate on the more sensitive characteristic curve.

Figure l is a diagram illustrating this protective system including the differential relay utilized in this invention arranged to obtain double sensitivity dependent upon the relative directions of flow of the currents in its differential windings.

Fig. 2 is a diagram illustrating the special impedance device utilized in this invention for maintaining a delinite ratio of control current distribution.

Fig. 3 is a graph illustrating the characteristic curves of the special relay when operated upon its double sensitivity characteristics.

Fig. Ll is a diagram illustratingr this pilotwire protective system of the circulatingcurrent type applied to a three-phase power system and adapted for both ground and line protection.

Figs. 5 to l0, inclusive, are diagrams, in schematic form, illusti ting' the distribution of current in the pilot-wire and control system under various conditions of faults.

F 5 is a diagram illustrating the characteristic distribution of the control currents under normal conditions, that is to say, with no fault on the section to be protected of the power system. It also illustrates the distribution of currents for the case of a balanced three-phase fault outside of the section shown.

Fig. 6 illustrates the characteristic distribution of control currents in case of a phase-to-phase fault with equal currents fed from both ends of the faulty power sect-ion.

Fig. 7 is a diagram illustrating the characteristic distribution of control currents for the case of a phase-to-phase fault with the fault current fed from one end only of the faulty section.

F 8 is a diagram illustrating the characteristic distribution of control currents for the case of a phase-to-phase fault with twice as much fault current fed from one end of the faulty section as from the other end. It also illustrates a reversal of the relay eurrents at the end of the section fed by the smaller magnitudes of fault currents whereby the a -angcmcnt autmnatically becomes more senr.. ive.

Fig. 9 is a diagram illustrating the characteristic distribution of control currents for the case of a phase-to-grouml fault within the faulty section and with equal fault currents fed from each end of the section.

Fig. l0 a diagram illustrating the characteristic distribution of control currents for the case of a` phase-to-ground fault on an adjacent section. It also illustrates the novel protection afforded by utilization of the neutral resistors or impedance devices.

By way of example. this protective system will bc described with reference to Figs. l and 4:.

The devices represented by l1 and l2 are station bus bars of a three-phase power system. The purpose is to furnish a source of main power to the power conductors A. B and C between the stations 11 and 12. The source of main current may be an alternatingcurrent power system of any type and, with modifications, a direct-current power system. The power system may be of the overhead type or of the underground type and may be a o'rounded or an ungrounded power system. This protective system may also be applied to the protection of the apparatus acting as a source of power, such as generators or transformers, but, by way of example, it will now be described with reference to a three-phase grounded power-transmission system only.

The device 13 is a circuit interruptor or circuit breaker of the usual type or means for isolating the main line, comluctors A, B and C from the rest of the power system.

The device la a current transformer hav ing its primary winding connected. in one of each of the main-line conductors A, B and C near the ends thereof and havingr the secondaries connected to a pilot-wire system to be described later. The devices il are current transformers of the usual type. They are preferably arranged, as shown, in star connection. The purpose of the transformers M is to supply current to the pilotwire system which, in magnitude and direction, is progortional to the currents in the main conductors to which they are respect-ively connected.

The relay 2l is preferably an induction type, alternating-current, balance relay comprised of two windings and 2l acting through a common magnetic circuit on a single disk 25, the direction of motion of which de iendent upon which of the currents in the two windings and Q4 predominates. The relay equipped with a torque compensator' or 5 duration transformer 28, as illustrated in l* l, and of essentially the same construction whether it be used as a line relay 2l or a ground rela.)v .12 (Fig. il). The relay operates on the induction principle, responsive to an unbalanced current or excess current between its two windings and 24. In effect, the relay has two overload elements 23 and 24 acting in opposition upon a common disk 25 through a common magnetic circuit. The two elements and 24 are electrically opposed and, under conditions of balanced current through its two windings, the fluxes in either the lower pole 29, or in the saturation transformer 28 of the relay are equal and opposite, giving a rcsultant zero torque on the relay disk 25. From the Figure 1, it may be seen that the windings 23 and 24 on the lower pole 29 are opposed and that the windings on the torque compensator' 28 are additive in effect, when each has a normal flow of current. The upper poles 27 are fed or energized from the torque compensator or the saturation transformer 28 which, at the same time, serves to give the relay a definite minimum time characteristic. The moving element 25 consists of a solid aluminum diskfastened to the same shaft as the moving contact of the two contacts 26.

Under the normal conditions of balanced currents flowing in the two windings 23 and 24, as shown in Fig. 5, for example, no flux is produced in the main pole 29 because the windings 23 and 24 balance each other, and, consequently, there is no torque upon the disk 25. If, however, the currents in the two windings 23 and 24 become unbalanced so that the current in the winding 24 exceeds that in the winding 23, suchdiifereutial current produces a flux in this main pole; and, by the interaction of this flux with the flux in the upper poles due to the current supplied from the saturation transformer 28, the operation of the relay results in closing of the contacts 2G.

The action is substantially the same regardless of the relative direction ofthe currents in the windings 23 and24, although the relay automatically becomes more sensitive for one direction of current relative to the other. When the direction of one relay coil current becomes reversed with respect to the direction of the other coil current, as is shown for the relays associated with the bus 12 in Fig. 8, the fluxes produced by the two currents will be additive in the main pole 29 and subtractive in the torque compensator 28. Since the value of ampere turns which will cause saturation of the main pole 29 is different from that which will cause saturation of the torque compensator' 28, the relay willoperate on different cha acteristics for the two cases. Curve A of Fig. 3 illustrates the sensitivity characteristic of the relay when the directions of currents are such as to be additive in the torque conniensator 28, while curve `B shows the sensitivity characteristic when the currents are subtractive in the torque compensator 28 and additive in the main pole 29.

This relay operates on rcurrent alone and no source of potential isrequired. This is upon the occurrence of an imbalance in current in the coils 23 and 24 produced by a fault connection relative to the main-line conductors.

The device 22 shown in Fig. 4 is a relay similar in construction to that of the device 21 already described except that it is a relay of more sensitive type and that it is connected to the ground pilot wire 54, as illustrated. The purpose of this relay is to permit greater sensitivity in operation upon the occurrence oit a ground fault, because it operates upon smaller imbalances or differences in the distribution of currents between the coils 23 and 24.

The devices 31 are resistors or impedance devices or means for placing resistance or impedance in the connections between the relays 21 and the pilot wires 51 to 54, inclusive. The purpose of the resistors is to cause the currents in the relays to divide equally between he two coils 23 and 24, under normal operating cenfflitions. The resistors are adjusted to have resistance equal to one-half the impedance of the pilot wire, when. the insulating transformers 41 are not utilized. and, when the insulating transformers 41 are utilized, the adjustments of the resistors 31 must be made to take care of the additional effect-s of the utilization of the insulating transformers upon the impedance of the pilot-wire system. ln broad terms, the devices 31 are means for balancing the distribution of currents between the coils 23 and 24 of the relays 21.

The device is a resistor or impedance device associated with the ground relay 22. Its purpose is to cooperate with the resistors 31 to balance the distribution of currents between the coils and 24 of the ground relay 22, under normal conditions, and to maintain the balance when a through fault occurs. No current flows in this resistor 32 in the absence of a ground fault or, in other words, the currents of a short circuit or of a phase-to-phase fault have no effect upon the resistor 32 or its associated relay 22. The principal purpose of the resistor 32 is to obtain greater sensitivity and selectivity in adjustments of the system for protection against a ground fault upon the main lines. This is a novel feature.

other two phases. By'comparing the current directions for this case with those shown by the arrows in Case One, it is apparent that the direction of current flow is now reversed ait one of the :two stations. Therefore, the voltages in the two ends of the pilot wire are opposed; and, since 'the Vcurrent-"transformer primary currents are equal, these opposing voltages are equal. As a result, no current can circulate Athrough the pilot wires; 'amd fthe 'total secondary current must flow through one coil of the relay 21. Consequently, the -relavs 21, at both ends of the section 11--12, will operate and be effective to open their respective circuit breakers 13. p

Clase III. (f'liaraczeri'stz'o distribution of currents for case of a phase-to-phase fault with fault currents fed from one end only The conditionsassumed for this case, which is represented by F'g. 7, may exist it the system is so connected that the power can iiow in -only one direction in the section 11-12. It will also exist if the circuit breaker 13 at one end of the section 11-12 is opened before 4the circuit breaker 13 at the other end ofthe section has opened. kIn other words, we may assume that, for Case III, the breaker 13 at the right hand end of 'the section 11-12 opens before the breaker at the other end opens. The condition shown in Case III will then exist.

For the case thus resulting, there will be yno voltages at the right hand end of the section -to affect the control-current distribution, and, consequently, the control-current division will be inversely proportional to the impedances of the two paths. It should be mentioned at this point that the impedances ot the relays have been neglected in our discussion of the several conditions assumed. 'Since these iinpedances are actually very low, they may be neglected without causing any appreciable diierence 'between these calculated current values and the values which would actually be measured by the use of amineters. Returning now to our discussion of 'Case III, we find that one of the two paths through which the current will flow consists ot two of the balancing resistors and the other consists of two resistors and two pilot wires. Consequently, the impedance of one path is only one-third of that of the other, the respective currents flowing will be in the ratio of three to one. Therefore, if we have a `total of l() amperes 'ilowing, the current in one coil 24 of the relay 21 will be 7.5 aniperes and the current in the other coil 23 will be 2.5 ainperes. Thus, an unbalance exists in the relay and it will operate or be effective to open its associated breaker 13,

It might appear at first glance that there would be paths for the current vthrough the ground pilot wire 54 and through the pilot vwire 51' corresponding to the phase in which 'these two additional shunt paths are yconnected between points of equal voltage, and,

therefore, no current will How in them.

It will also be noted that an unbalanced condition exists only in `the relays 21 at the end 11 of the section 11-12 from which power is being fed to the fault point. The currents in the relay coils 23 and 24 at the other end lof the section 11--12 are equal, although the Arespective direction of one, as referred `to the other, is reversed. However, sin/ce the relay 21 consists of two separate coils .23 and 24 acting` upon an induction disk 25, the torques tending to rotate this will still be equal and opposite in direction. lConsequently., -'the disks 25 will not rotate, and the breaker at the end 12 ot' the section 11-12 will not be opened. 'This does not constitute an objection to this protective systeni, however, since, if there is no fault current ted troni thisend l2 or' the section, there is no need for this breaker to open.

Oase IV. )'z'stribuz0n 0f @arrears /Or t/ifc case of a, ,rilascio-phase fault n the sectto-n wit/i unequal ffl-u2?? current fed from the ifi/.i cada Case IV, which represented by F ig. 8, illustrates the current distribution for a condition intermediate between those described in Cases II and ITI (Figs. 6 and 7). For the sake of convenience, it has been assumed that twice as much fault current is being fed from one end of the section 11-12 as from the other. The actual control current distribution will be similar to that shown in the diagram, Fig. 8, but since for this case, there are opposing voltages of unequal niagnitudes at -the two ends of the section 11-12, the distribution will be affected by these voltages. Since these voltages depend upon the characteristics of the current transformers 14, the exact distribution cannot be deterinined unless these characteristics are known. Thevalues shown on the diagram may be considered as representative of an average case, however.

This case will serve to illustrate an lirnpoitant characteristic of the relay 21 which is advantageously utilized in this scheine. -By referring to the diagram, Fig. 8, it is apparent that unbalanced conditions exist in the vrelays 21 at both ends of the section 11 12 and that the unbalance is Vgreater vat the end 11 of the section froin whichk relays 21 are in the same direction, while,

at the other end, the currents are in opposite directions, in any one relay. As a result of this, the relays 21 at the tivo ends 11 and 12 of the section Will operate in accordance with diiierent characteristic curves. This may be clarilied by reference to Figures 1 and 3. in Fig. 8, the control currents at the end 11 are in the same relative direction, and, at the end 12, they are in opposite directions. Fig. 3 shows the characteristic operating curves for the relay 21. These curves are plotted 7with the smaller of the tivo coil currents as abscissa and with the amperes imbalance to close contacts as ordinate. The current values shown apply for only one tap setting of the relay, but, by multiplying the values by the proper constants, the same curves could apply for any tap setting. It is evident from these curves that, When the relay is operating in accordance with curve A, a larger unbalanced current is required to close the relay contacts 2G than When it is operating in accordance with the curve B. Then the current directions are as shown at the end 11 in Fig. S, the relay will operate with the characeristics indicated by the curve A, and, when 'the current directions are as shown at the end 12 in Fig. 8, the relay will operate in accordance with the curve B. n the diagram of current distribution, which is Fig. 8, it Will be evident that the relavs 21 at the end 11 ot the section Which is feeding the greater amount oit current to the fault Will operate with t-he characteristics shown by curve A of Fig. 3. While the relays at the end 12 of the section from which the smaller fault current is being fed Will operate With more sensitive characteristics, as shown by curve B. Since the unbalance existing in the relays is less at the end 12 of the section which :feeds the smaller fault current, it is very desirable that the relays 21 at this end 12 should have the more sensitive characteristics. This feature of the relay, therefore, is utilized very advantageously in this system of protection.

Oase V. Distribution 0f currents for cmg/Zep/zcsc-o-grotmcZ-faialt wit/tin the section (mtl with egual currents fccl from cach @mi This case is represented by Fig. 9. The neutral balancing` resistor 32, which is a novel and important feat-ure of this protective system, has not entered into the four cases previously described. To illustrate the eii'ect ot this neutral balancing resistor 32 upon the control current distribution, tvvo cases of ground faults will be described. Theiirst case will be that of a single-phase-to-ground fault Within the section 11-12 with equal currents 'fed from both ends, and the second case (de` scribed under Case VI, Fig. 10) will be that ot a similar fault fed Within an adjacent section, i. e., a through fault.

Since the voltages at the tiro ends of the ground pilotl Wire 511 and of the pilot wire 53 for the faulty phase are equal and opposite in direction Ytor the case assumed above, no current will lion' in either of these pilot wires. Consequently, the total secondary current vfill flow through only one coil 24 of the relay 2l, and the breakers at both ends 'will open. As was previously stated, a ground relay 22 having lower current tap settings may be used on systems Where the ground-fault current may be small or less than the minimum tripping value for the line relays.

@use VI. Distribution of currents for case of c .Sz/1, g/c-p/iasc-to-g/'01u/1d in im (affirment i.. Y acciao/a.. /Scc 112g. l0.

The value ot' the neutral balancing resistor 32 will be apparent trom the consideration ol' a fault in an adjacent section. By refer ring to the diagram, Fig. 10, it will be seen that the current divides so that the relays at both ends or the section .1l-l2 remain balced. It the neutral .Stor 52 were omitted, the currents would no longer divide equally between the two relay coils 23 and 2l and an imbalance would exist which would cause operation ot the relays for a fault external to the section. Any attempt to adjust the resistors 31 in the individual phase-s in order to balance the relays for the through-fault condition assumed would result in an undesirable distribution of current tor other fault conditions and would make it necessary that the sensitivity of the relays be decreased in order to prevent incorrect operation.

li the neutral balancing resistor were omitted, the resistance of each of theremaining resistors 3l would have to be doubled in order to retain balanced conditions during an externa phase-to-grouiul fault. As a result of this, tn'ice as much current would iloiv over the pilot u'irefs as through the balancing resistors under normal operating conditions. This would not cause incorrect operation but would place a resultant restraining torque on the relay which would decrease its sei vivity. That is, in order to cause operation, or to cause the contacts to close in the same time for both cases, the Jfault would have to be more severe it there were an over-balanced ref-training torque on the relay under normal conditions. By utiliaing the neutral balanci g resistor 32, the restraining torque on the relays 21 may be maintained more nearly in balance with the operating torque and no over-balaiicinUf, with its consequent loss of ser itivity, will be required to maintain the effectiveness oi the relay system under all possible conditions ot through7 faults. The neutral balancing resistor 32 is, therefore, an in'iportant element in this scheme, and is essential to a correct and sensitive operation.

lVhen a through faultcurrent flows in the section 11-12 produced by a phase-tophase fault in some other section, a circulating current flows in two of the phase pilot wires of the section 11-12 in the phases corresponding to the phase location of the fault, and the circulating control currents thus produced are circulated by a total of four current transformers 14 over the two phase pilot wires affected. But, when a phase-toground fault occurs in some other section of the power system, a relatively smaller value of current will flow in the faulty phase wire or conductor by reason of the relatively large impedance that is usually in the circuit under a ground-fault condition. The ground fault produces a circulating current in the section 11-12 between one phase pilot wire and the ground pilot wire and is propelled by only two current transformers 14 instead of the four current transformers, as was the case in the phase-to-phase fault.

Therefore, by reason of this difference in magnitude between the phase-to-phase through fault and the phase-to-ground through fault and by reason of the fact that the effects of the former are transferred to the control system by cooperation of double the number of current transformers, a distinction must be made in the two types of through faults and their effects upon the non-faulty section 11-12.

Heretofore, some of the circulating-curront pilot-wire systems have been able to give correct balance or equal sensitivity for only one type of fault, i. e., either the phaseto-phase or the phase-to-ground type, but this protective system, by the utilization of the special impedance device 32 connected into the neutral, compensates for both types of through faults.

The purpose of the balancing resistors 31 and 32 is then essentially not only to s-plit up the control currents with a predetermined definite balanced ratio between the two windings of the relay 21 but also to maintain that ratio when a ground fault occurs in some section of the power system other than in the non-faulty section, which by this protective system alone, is permitted to remain undis- 1 turbed.

One advantage of this protective system is that the volt-ampere burden upon the linecurrent transformers 14 is unusually low. The fundamental reason for the reduced volt- `ampere burden on the line-current transformers 14 is that the controlcurrent from the transformers 14, divides in the relays 21 associatedwith them, one-half only of the control current passing out normally over the pilot wire, whereas, in the protective systems In this system, however, since the pilot wire carries substantially only one-half of the current from the transformer 14, it imposes only one-fourth the burden on the transformer, since, from the formula above, the voltampere burden varies as the square of the current. The balancing resistor imposes a like burden, thus making the total :'lr 0I of the burden imposed by a circulating-current system which does not employ this divisional-current scheme.

Another advantage is that high voltages across or in the secondaries of the current transformers 14 are avoided, because this particular system is a circulating-current system and further because of the utilization of the balancing resistors 81 and 32.

Another advantage is that the relays 21 may be so adjusted or set that an open circuit in the pilot-wire system will not cause a false operation and that only a fault condition will cause an actuation of the circuit interrupters. An open circuit in the pilotwire system will not cause a false operation because the relay is so designed that, when only one coil is energized, the current required to cause the contacts to close must be equal to twice the tap setting used. Consequently, the relay may be so set that the normal load currents will not cause operation even though all of the current must flow through one coil of the relay, that is, the coil 23. However, this is not detrimental to the sensitivity of the relay because the relay will operate very rapidly at high values of current, such as would obtain on the occurrence of a fault.

Another advantage is that the balancing resistors 31 and 32 afford a means of obtaining a perfect balance of the relay system for any type of fault occurring outside the protected section, whether it be phase-to-phase or phase-to-ground, and that, to attain a satisfactory degree of balance, the resistors 31 and 32 need not be so precisely adjusted as in other pilot-wire schemes heretofore used. The reason that the resistors need not be adj usted so precisely is that small unbalances in the relay currents will not cause operation when normal load current is flowing. c

Another advantage of this protective arrangement is that the current transformers 14 need not be exactly similar in characteristics at the two ends of any one line to be protected. Such differences in characteristics may be compensated for by adjustment of the balancing resistors 31 and 32.

Another advantage is that potentials induced in the pilot wires because of their proximity to the transmission line or because of lightning disturbances, or from other causes, will not cause false operation of the circuit interrupters 13, because the currents produced by such potentials must flow in series through the two coils of the relay. The current in one coil is then necessarily equal to the current in the other coil, and thus there is no unbalanced current and no actuation of circuit intcrrupters.

Another advantage is that this protective arrangement automatically becomes more sensitive to fault currents at the end of a 'fault ction in .vhicl flows the lesser value ot' such fault current. rlhis is accomplished by utilization ot' the double-sensitivity relavs which alter their sensitivity in accordance with the relative directions oli' the low ot circulating current in their windings.

Another advantage is that the balanced division ol.2 currents in a non-faulty section remains undisturbed when a through-fault current, prod ced by any type of 'fault in some other section of the power system, passes through the non-faulty section. This is accomplished by utilization of the neutral balancing resistors which produce this result not iuconiplishcd by systems heretofore used.

Another advantage is that this protective system may be applied to relatively long lengths oi power lines because the relay circuits have impedances of relatively small magnitude by reason of the fact that a part only and not all the current circulates over the relatively long pilot wires, the remaining part circulating over the relatively short balancing circuits in the local station.

I claim as my invention:

l. The combination in a pilot-Wire protec Uive system ol' the circulatinU-current type utilizing a ditllerential relay, I' a pilot wire, a relay balancing circuit including a neutral circuit, means for normally divi-ding the circulating current between the pilot Wire and the relay balancing circuit, and a special impedance means connected to the neutral circuit.

ln a protective system, the combination vith a power line, circuit interrupters for isolating the power line, actuating means for the circuit interrupters, a pilot wire,'a local balancing circuit containing a neutral cir* cuit and a relay responsive to excess current in the balancing circuit relative to that in the pilot wire for controlling the circuit in.- terrupter. ol means connected to the balanc- '1 ing circuit for normally balancing the currents in the balancing circuit against those in the pilot wire comprising an impedance device associated with the relay and another special impedance device connected in the neutral circuit.

3. In a protective system, the combina-tion with a power line, circuit interrupters for isolating the line, actuating means for the circuit interrupters, means for obtaining current proportional to the line current, a

pilot wire associated with the li ne and a local balancing circuit of a normally balanced de vice connected to the balancing circuit and the pilot wire and responsive to excess current in tl e balancing circuit ior controlling its associated circuit interruptor and means 'for imparting to the normally balanced dcvice a dual sensitivity depending upon the relative directions ot the l'low of current in the balancing circuit and in the pilot wire.

al. In a protective system ot' the circulatingcurrent pilot-wire type, the combination with a power line having circuit interrupters for isolating the line, actuating means for the circuit interruptcrs, means for obtaining a pilot wire current proportions.i to the line current, a pilot wire associated with the line and a local balancing circuit, of means for normally dividing the pilot wire currents equally between the pilot wire and the balancing circuit comprising a line or phase impedance device connected in series-circuit relation with a neutral impedance device, and means responsive to an excess current in the balancing circuit for controlling its associated circuit interruptor'.

ln a protective system, the combination with a power line or conductor in a power system having circuit interrupters adapted, when operated, to isolate or cut out the power line, and current transformers having their primaries connected to the power line and their secondaries connected to a pilot-wire system ot' the circulating-current type, of a relay comprising an operating winding re sponsive to an imbalance in the division of the pilotwire current for controlling its associated circuit interruptor and a restraining winding connected to its associated pilot wire ilor opposing the actuation of the circuit interruptor, and means comprising a line impedance device and a neutral impedance device connected in series relation with each other for balancing the effects of the relay operating coil to norinally substantially equalize the effects ot' the relay restraining coil.

o. ln a protective s stem, the combination with a power line, circuit interrupters adapted, when operated, to isolate the line from the rest oli the power vtc1n, current transformers Aconnected to each conductor 0f the power line near both ot its ends, their secondaries being connected in star connection with the neutral connection brought out, a ground pilot wire connected to the neutral connection, a pilot wire associated with each conductor, a relay for controlling its associated circuit interruptor arranged normally to equally divide the currents from the current transformers between the pilot Wire and a balancing circuit and means in the balancing circuit for balancing the divided currents, of an impedancedevice connected between the neutral connection of the current transforniier secondaries and neutral connection of the last named means.

7. In a protective system, the combination with a section of a power' line having a plurality of conductors, circuit breakers adapted, when operated, to control the connections of the power line to the rest of the power system, means having a neutral connection for obtaining a control current proportional in magnitude and direction to that in the power conductors, pilot wires associated with the line conductors, balancing circuits associated with each conductor, and means associated with each conductor for balancing the division of control currents between the balancing circuits and the pilot wires, of a special impedance device associated with the neutral connection and means responsive to the division of control currents for controlling the circuit interrupters.

8. In an electrical system, the combination with two interrupters each at opposite ends of a section and current transformers associated therewith for each phase conductor, of a differential double-winding relay, means including a conductor for connecting one Winding in a relay at one end to the corresponding winding in the same phase relay at the other end of the section, and means including a resistor for connecting the other winding of each relay in a local circuit with its current transformer, each resistor being substantially equal to one-half of the pilot wire impedance, and a special impedance device connected to the neutral of the current transformers and to the neutral of the resistors.

9. In a protective system, the combination with a section of a polyphase power system, means in each phase for attaining a control current proportional to each phase current at each end of the section, a phase pilot wire and a balancing circuit associated with each phase and a ground pilot wire, of means for splitting the control current in each phase with a definite ratio between the phase pilot wire and the balancing circuit, and means for maintaining that ratio substantially constant when a through fault occurs.

10. In a protective system, the combination with a section of a polyphase power system, means in each phase for obtaining a control current proportional to each phase current at each end of the section, a phase pilot wire and a balancing circuit associated with each phase and a ground pilot wire, of means for normally dividing equally the current effects between the phase pilot wire and the balancing'circuit, and means including a neutral balancing resistor for maintaining the division constant when a through fault occurs.

11. In a protective system for a section of a polyphase transmisison circuit adapted to transmit currents normally balanced between the phases of the circuit, relays at both ends of each phase of said section responsive to an unbalance of the normally balanced phase currents, means, including pilot conductors, for transmitting currents proportional to the currents in each phase and the residual current resulting from said imbalance, a relay at each end of said section responsive to said residual current, and means controlled by said relays for isolating said section.

12. The combination with a pilot-wire protective system associated with a power line in a power system, of a rela-y means for controlling the connections of the line to the rest of the power system, actuating and restraining windings for said relay means, means for normally dividing the circulating current between said windings, and means for imparting a dual sensitivity to said relay means depending upon the relative direction of current flowing in the two windings.

In testimony whereof, I have hereunto subscribed my name this 19th day of November,

HERBERT C. GRAVES, JR. 

